During a printing operation, for the purpose of process monitoring, it is usual to provide printed control strips with colored test patterns outside the subject on sheets or webs to be printed. These control strips, whose longitudinal direction is transverse with respect to the transport direction of the printing material, contain a set of measurement areas on each of which a specific characteristic variable characterizing the printing quality can be measured, and which are typically repeated periodically in the longitudinal direction but do not have to be.
A typical configuration is, for example, in the case of four-color printing, a sequence of measurement areas which are printed with the full-tone colors black, cyan, magenta and yellow, in order to permit a measurement of the respective ink density. By using such ink density measurements, conclusions about the ink supply in the inking unit of the press can be obtained, and its setting can be optimized when starting up the press but also during continuous operation.
For measurements of this type, it is known to arrange a measuring apparatus in a press which comprises a light source for illuminating the printed product, a camera aimed at the printed product for recording an image of an extract of the printed product, and an electronic evaluation unit. In this case, the aforementioned extract of the printed product has a control strip of the type mentioned previously. The camera has an electronic two-dimensional image sensor, from which the evaluation unit reads the images acquired and from these determines characteristic variables of the printed product, for example density values of the printed inks. Apparatuses of this type and methods for their operation are described in EP 0 741 032 B1 and DE 195 38 811 C2.
In such measurements, the intensity of the light reflected by a printed product always depends on the intensity of the light shone in by the illumination device. For this reason, there is always a need for appropriate equalization of the measuring device. For this purpose, the intensity of the reflections of the unprinted printing material, which is referred to as a “white reference” in the following text, is recommended as a reference variable. In the case of ink density measurements, the intensity of reflection from a printed area is in any case placed in a relationship with that of the unprinted printing material, so that the latter has to be acquired as a reference variable.
In a measurement on an elongated control strip, which extends over virtually the entire width of the printing material, inhomogeneities of the illumination and of the camera sensitivity over the length of the image to be acquired cannot be disregarded. In order to avoid measurement errors as a result of these inhomogeneities, a local white reference is necessary. One such method is taught by DE 195 38 811 C2 in the form of recording an image of an unprinted area with a size covering the dimensions of a control strip. However, faults arise in the event of variation of the illumination intensity over time. In this regard, the aforesaid document mentions as a countermeasure the use of unprinted areas within the control strip as a white reference but without making any statement as to how such a use is to be understood.